The present invention relates to a glow plug used to preheat a sub combustion chamber or a combustion chamber of a diesel engine and, more particularly, to a self-temperature control glow plug of a ceramic heater type, which has a rapid heating property and a self-temperature control ability and can achieve afterglow over an extended period.
Of conventionally known diesel engine glow plugs having various structures, ceramic heater type glow plugs have recently attracted attention because, for example, they can function as rapid heating type glow plugs. U.S. Pat. No. 4,401,065 discloses an example of such a ceramic heater type glow plug using a ceramic heater in which a heating wire consisting of tungsten (W) or a rhenium alloy (Re) is buried in an insulating ceramic material. A glow plug of this ceramic heater type is superior in thermal conduction coefficient to a glow plug of a conventionally common sheath type. Therefore, a glow plug of this type improves its heating characteristic or temperature rise characteristic, thus acquiring performance as a rapid heating type. In such a glow plug with a ceramic heater, however, one type of heating wire is simply buried in the heater. Therefore, it is difficult to obtain, with this single heater element alone, a self-temperature control ability of controlling the heating characteristics of the heating wire such that a predetermined heating characteristic and a predetermined saturation temperature characteristic are obtained. As a result, a resistance wire or the like for power control must be additionally provided on a power supply circuit.
For this reason, U.S. Pat. No. 4,650,963, for example, has already proposed a glow plug in which in a metal holder constituting the glow plug, an auxiliary heater consisting of a metal sheath, which incorporates a resistance element capable of controlling power supply to a ceramic heater, is connected in series with the rear end of the ceramic heater.
In the glow plug having the above structure, however, it is necessary to use the sheath type auxiliary heater in addition to the ceramic heater. The results are an increase in the total number of parts and problems in the manufacture of the glow plug, which lead to a high cost. In addition, placing the sheath type auxiliary heater for power control in the metal holder of the glow plug increases the temperature inside the holder although the holder requires no temperature rise, resulting in a problem of mechanical strength against heat in individual parts. This inevitably poses problems such as wasteful power consumption as well as poor reliability.
Another ceramic heater structure is proposed in, e.g., Japanese Patent Laid-Open No. 58-10919, in which two types of metal wires with different positive temperature coefficients of resistance are connected in series with each other and buried in an insulating ceramic material. However, if silicon nitride, for example, is used as the insulating ceramic material in putting such a structure into practical use, it is difficult to select a metal wire having a positive temperature coefficient of resistance small enough to resist the sintering temperature of the material, approximately 1,800.degree. C. Therefore, a consideration must also be given in this respect in adopting the above structure.
In particular, a glow plug of this type has been recently, strongly required to employ a so-called afterglow system. In this system, power supply to a glow plug is maintained for a predetermined time period after an engine is started, thus smoothly and properly performing a combustion inside the engine. In addition, it is required to increase this afterglow time as long as possible. In performing power supply control during afterglow, the conventional ceramic heater as described above has problems in durability such as mechanical strength against heat. A demand has therefore arisen for some countermeasure made in consideration of these problems and capable of achieving a function as rapid heating and a self-temperature control function, and simplifying the overall structure.